Conventionally, wireless tire condition monitoring apparatuses have been proposed that permit the driver in the passenger compartment to check the conditions of the tires of a vehicle. One such tire condition monitoring apparatus includes a receiver mounted in a vehicle body and transmitters attached to respective wheel assemblies. Each transmitter detects the condition of the associated tire, that is, pressure and temperature in the tire, and wirelessly transmits a signal containing data that represents the detected tire condition. The receiver receives signals transmitted from the transmitters through a reception antenna, and displays information regarding the tire conditions on a display provided in the passenger compartment as necessary.
In such a tire condition monitoring apparatus, the receiver is preferably configured to determine which one of the wheel assemblies a received signal has been transmitted from, in other words, the position of the wheel assembly associated with the received signal.
In the tire condition monitoring apparatus disclosed in Patent Document 1, a transmitter is attached to each of the wheel assemblies of a vehicle. The transmitters transmit signals (radio waves). A receiver is provided in the vehicle body. The receiver has a reception antenna, which is arranged at different distances from the respective transmitters. The receiver receives signals transmitted from the transmitters. Signals transmitted from the transmitters are attenuated before reaching the reception antenna. Thus, the greater the distance from the reception antenna to the wheel assembly, which has a transmitter, the weaker the received signal strength indication, or RSSI, of the signal received by the receiver becomes. In Patent Document 1, the receiver identifies the positions of the wheel assemblies based on the magnitude relationship of the differences between the maximum values and the minimum values of the RSSIs of the signals transmitted from the transmitters. For example, the receiver determines that the ascending order of the magnitudes of the difference between the maximum value and the minimum value of the RSSI agrees with the ascending order of the distances between the reception antenna and the respective transmitters that have transmitted the signals. Based on the positional relationship with the transmitters, the receiver determines the positions of the wheel assemblies, each of which has a transmitter.